Geoscience Reference
In-Depth Information
4. Discussion
All three models are generally consistent with the global-scale observations,
Table 1, and the aerosol and P2004 models achieve comparably small col-
umn abundances for O
2
. The calculated O
2
column abundances are larger
than the standard interpretation of the observational upper limit.
5
How-
ever, the correct way to compare the calculated O
2
with the observational
upper limit is via a radiative transfer model simulation of the spectroscopic
observation. Prior radiative transfer simulations
33
suggest that if the O
2
profile has the shape predicted by the photochemical models, then an O
2
column abundance which is smaller than about 3
10
18
cm
−
2
×
is roughly
consistent with the observational upper limit. This factor of
two uncer-
tainty in interpretation of the observational upper limit arises because mul-
tiple scattering within the upper cloud strongly enhances absorption by
O
2
within the upper cloud and the O
2
profile assumed for the standard
interpretation of the upper limit observation
5
has the vast majority of its
column abundance within the upper cloud. Most model calculated pro-
files, however, are severely depleted in O
2
below 70 km altitude, Fig. 1.
In addition, the observational upper limit was derived from observations
of a portion of the day side atmosphere. O
2
airglow and CO abundances
vary significantly across the night side, so the current upper limit on O
2
may not be representative of the global-average O
2
calculated in the pho-
tochemical models. If the O
2
abundance is below the present upper limit
then a combination of all three catalytic mechanisms discussed here may
be required.
The primary disagreement between current models and observations is
in the calculated global-average CO profiles. However, no CO observations
have been reported for the day side at 60-90 km for the past two decades
and none of the extant observations provide day side profiles.
Venus Express
should help fill this gap.
The three catalytic mechanisms investigated are most effective at differ-
ent altitudes, Fig. 2: Reaction (1) above 90 km, Reaction (2) below 80 km,
and the gas-phase chlorine catalytic mechanism at 65-90 km. As photodis-
sociation of CO
2
occurs primarily at 70-100 km, the latter two mechanisms
severely deplete oxygen and CO in the upper cloud. Consequently, a source
of CO at the base of the upper cloud, which was photodissociation of OCS
in our models, is required to explain the CO abundance observed in the
upper cloud.
19
∼
